1. Field of the Invention
The present invention relates to a battery charger for charging a secondary battery and a charge control circuit for controlling the charging operation.
2. Description of the Related Art
A secondary battery is used in a variety of equipment, such as a note-size personal computer, a communications equipment, a video camera, etc. Therefore, as a natural consequence, a battery charger for charging these secondary batteries has been widely used.
When a secondary battery is charged, voltage to be applied to the secondary battery or current to be supplied to the secondary battery is usually used as a parameter for controlling the charging operation. As the prior art, for example, there is a method in which a charging operation is divided into two processes. In the former process a secondary battery is charged almost up to the full-charged state while supplied current is maintained constant, and in the latter process the secondary battery is charged until the full-charged state while supplied voltage is maintained constant, is widely known.
FIG. 1 shows an example of a conventional battery charger. This battery charger is generally called an "asynchronous rectification type" battery charger.
A voltage detection unit 101 detects output voltage (voltage applied to a secondary battery 200). A current detection unit 102 detects an inductor current or its average value (current supplied to the secondary battery 200) by monitoring a voltage generated across a resistor R connected in series to an inductor L. A charging control unit 103 controls a switch (MOS transistor) M1 based on the output voltage detected by the voltage detection unit 101 and the inductor current detected by the current detection unit 102. Here, if it is assumed that the switch M1 is driven by a PWM (pulse width modulation) control method, a control signal supplied to the switch M1 is a pulse signal with a specific frequency. The charging control unit 103 adjusts the duty cycle of the pulse signal based on the output voltage and the inductor current.
A prescribed DC voltage generated by an AC adaptor (AC/DC converter) is supplied to the switch M1. Therefore, while the switch M1 is ON (closed), as shown in FIG. 2, the inductor current increases as time elapses. During this period, the inductor current is supplied via the switch M1. While the switch M1 is OFF (opened), the inductor current decreases as time elapses. During this period, since the switch M1 is OFF, the inductor current is supplied via a diode D1.
In this circuit, the charging control unit 103 controls the switch M1 in such a way that the output voltage is maintained constant or in such a way that the average inductor current is maintained constant. As a result, the secondary battery 200 is charged with a constant voltage or a constant current.
FIG. 3 shows another embodiment of a conventional battery charger. This battery charger is generally called a "synchronous rectification type" battery charger.
The synchronous rectification type charger shown in FIG. 3 can be basically implemented by replacing a diode D1 with a switch M2 in the charger shown in FIG. 1. The switches M1 and M2 are synchronously driven in such a way that the switches are not simultaneously turned on. Here, when the inductor current flows in a reverse direction, the current detection unit 102 cannot detect the inductor current correctly, if it does not have a function to detect a negative inductor current. Therefore, this battery charger has a diode D2 to prevent the reverse inductor current.
The operation of this synchronous rectification type battery charger is basically the same as that of the asynchronous rectification type one shown in FIG. 1. Specifically, while the switches M1 and M2 are ON and OFF, respectively, the inductor current increases as time elapses. During this period, the inductor current is supplied via the switch M1. While the switches M1 and M2 are OFF and ON, respectively, the inductor current decreases as time elapses. During this period, the inductor current is supplied via the switch M2.
In the above circuit, the charging control unit 103 synchronously controls the switches M1 and M2 in such a way that the output voltage is maintained constant or in such a way that the average inductor current is maintained constant. As a result, the secondary battery 200 is charged with a constant voltage or a constant current.
In the asynchronous rectification type battery charger shown in FIG. 1, as described above, the inductor current is supplied via the diode D1 while the switch M1 is OFF. However, the on-resistance of a diode is generally fairly large compared with that of a MOS transistor, etc. Therefore, when current flows through the diode D1, there is fairly large voltage drop, and as a result, a large amount of heat is generated and wasted.
In the synchronous rectification type battery charger shown in FIG. 3, the heat problem of the diode D1 can be solved by replacing the diode D1 with a MOS transistor. However, since the diode D2 is provided, a large amount of heat and an extra voltage drop on a charging route are generated by the diode D2, which is a new problem. In addition, since the reverse inductor current is prevented in conventional battery chargers shown in FIG. 1 and FIG. 3, there are following problems.
When the secondary battery 200 is charged up to almost full-charged state, charging current is controlled by the charging control unit 103 to decrease in such a way that the output voltage can be prevented from rising beyond a target value. As a result, the average inductor current becomes small or almost 0(zero). At this time, if a prevent function which prevents a reverse inductor current is not provided, the waveform of the inductor current becomes continuous as shown in FIG. 4A. However, if the prevent function is provided as shown in FIG. 1 or FIG. 3, a period in which a charging cycle is not performed is needed in order to make the average inductor current small or almost 0(zero) and to prevent the output voltage from rising beyond the target value. In this case, as shown in FIG. 4B, charging cycle is performed discontinuously. Therefore, the waveform of the inductor current becomes discontinuous, as shown in FIG. 4B.
In other words, if a prevent function which prevents a reverse inductor current is not provided, the secondary battery 200 is always charged with a constant cycle, regardless of the charging state of the secondary battery 200. However, if the prevent function is provided as shown in FIG. 1 or FIG. 3, the charging cycle changes according to the charging state of the secondary battery 200.
In addition, since the inductor current can be more easily averaged when an inductor current waveform to be detected is continuous than when an inductor current waveform to be detected is discontinuous, the detection accuracy of the average inductor current generally becomes low if the prevent function is provided. Therefore, if the target value of a charging current is set to a value lower than the ripple of the inductor current while the prevent function is provided, an inductor current waveform becomes discontinuous. Accordingly, the detection accuracy of the average inductor current decreases and there is a possibility that the deviation from the target value of the charging current may become large.